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| Home > Publications database > Methan im nördlichen Westsibirischen Becken: Bildung, Lagerstättendynamik und Austausch mit der Atmosphäre |
| Home > Publications database > Methan im nördlichen Westsibirischen Becken: Bildung, Lagerstättendynamik und Austausch mit der Atmosphäre |
| Book/Report | FZJ-2019-02111 |
1997
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/21985
Report No.: Juel-3412
Abstract: This work is part of the Russian-German scientific project entitled "Generation of natural gas in the West Siberian Basin and balance of atmospheric methane". It is concernd with the generation of natural gas (methane), accumulation of gas within the reservoirs in West Siberia, and with measurements of the methane exchange between soil/lithosphere and atmosphere. Based on compositional data and isotope geochemistry natural gas in northem West Siberia can be divided into three groups. These are: natural gas in Jurassic rocks, natural gas in Neocomian rocks and natural gas from the Aptian to Cenomanian Pokur Formation. Natural gas in Jurassic rocks was generated thermogenically from rocks of the Jurassic Tyumen Formation. Natural gas in Neocomian rocks is also of thermogenic origin, possibly being generated from the organic matter of Lower Cretaceous sediments. The largest accumulation of natural gas occurs in sands tone reservoirs in the Pokur Formation. This gas can be described as a mixture between thermogenic gas from deeper strata and isotopically light almost pure methane. 98.6 % of this gas consists of methane with an unusual isotope signature of -51.2 °/$_{\circ \circ}$. It is not possible to explain the existence of this methane with established concepts of gas generation.A new model was developed to examine the possibility of a thermogenic origin of the isotopically light methane in early mature rocks of the Pokur Formation. Based on pyrolysis experiments and reaction kinetic calculations the model enables the simulation of stable carbon isotope ratios of hydrocarbon components in natural gas. The temperature dependent kinetic isotope fractionation is defmed by a difference in the activation energies of $^{12}$C- and $^{13}$C-methane generation. The application of the new method to two coaly sandstones of the Pokur Formation results in a good correspondence between modelled carbon isotope ratios and $\delta^{13}$C values of methane in the reservoirs. The mass of methane thermogenically generated within the Pokur Formation under the gas field structures, however, is not sufficient to explain the mass of accumulated methane. Based on press ure and temperature data, porosity and permeability data, and the chemistry ofgroundwater a model of the 2600 m thick Cretaceous aquifer under the Urengoj gas field was developed. The possible influence of degassing of the groundwater on the accumulation of natural gas was quantitatively estimated. The most important mechanism for the degassing of groundwater was the 800 m uplift of the northem part of the West Siberian Basin beginning in the Late Eocene. This process can quantitatively explain the largest accumulation of natural gas in the world (Urengoj field) by degassing the methane from groundwater. The groundwater in the Cretaceous aquifer flows from south to north. Thus, the methane that was liberated from aqueous solution under the trap must have been generated in parts of the basin located south of the recent gas accumulations. Possible sources of methane in aqueous solution are the early thermogenic generation in early mature Cretaceous rocks, the gas caps of the oil fields in the central part of the basin, and bacterial methane. During two field trips in the Urengoj - Tarko-Sale area the methane exchange between soil/lithosphere and atmosphere was measured using a dosed chamber technique. The detected methane exchange rates were not affected by the accumulations of natural gas, nor by the permafrost situation and destablilisation of possible gas hydrates. In the study area the natural methane exchange with the atmosphere is caused by bacterial methane generation in wetlands and bacterial methane oxidation in soils. The average emission from swamps and mires was 83.1 ± 64.5 mg m$^{-2}$ d$^{-1}$ (CH$_{4}$). Mineral soils acted as sinks for atmospheric methane with an average rate of -3.0 ± 2.3 mg m$^{-2}$ d$^{-1}$ (CH$_{4}$). A balance of the natural methane exchange with the atmosphere in the Urengoj area resulted in a net emission of 4.1 ± 3.4 · 10$^{6}$ g km$^{-2}$ a$^{-1}$ (CH$_{4}$).
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